1. Field of the Invention
The present invention relates generally to light emitting devices, and more particularly to light emitting devices with highly reflective properties for improved light output.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light and generally comprise an active region of semiconductor material sandwiched between two oppositely doped layers of semiconductor material. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
LEDs can be fabricated to emit light in various colors. However, conventional LEDs cannot generate white light from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). The surrounding phosphor material “downconverts” the energy of some of the LED's blue light which increases the wavelength of the light, changing its color to yellow. Some of the blue light passes through the phosphor without being changed while a portion of the light is downconverted to yellow. The LED emits both blue and yellow light, which combine to provide a white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
In recent years, there have been dramatic improvements in light emitting diode technology such that LEDs of increased brightness and color fidelity have been introduced. Due to these improved LEDs, lighting modules have become available to further increase luminous flux output. Both single and multi-chip modules have become available, with a single-chip module generally comprising a single package with a single LED. Multi-chip lighting modules typically comprise a single package with a plurality of LEDs. These lighting modules, particularly the multi-chip modules, generally allow for high output of light emission.
However, the emitted light from the device chip(s) may be largely non-directional and non-uniform, which can negatively impact the emission and optical efficiency of a lighting module. Often, a light diffusion lens, light scattering particles, and/or phosphor particles are disposed over the chip(s) to assist in achieving more uniform light emission. A fraction of brightness can be lost when utilizing such means, largely due to back-emission from the emitter, or scattering and back-reflection of light from a light diffusion lens, light scattering particles, and phosphor particles.
To redirect the back-emitted, scattered and/or back-reflected light, reflective materials have been disposed on the substrate of various light emitting devices. The reflective materials may be disposed on only portions of the substrate, or may be disposed as a reflective layer on the substrate. In other attempts to redirect scattered and/or back-reflected light, light-reflective, white printed circuit board (PCB) and/or substrate technology has been developed. The materials used for this existing technology are generally epoxy-based. Epoxy contains free radicals that may yellow during prolonged use and/or common fabrication steps known in the art, such as reflow soldering. Epoxy materials may also degrade in the presence of blue light.